Access Traffic Steering, Switching and Splitting - Low-Layer

Description

ATSSS-LL (Access Traffic Steering, Switching and Splitting - Low-Layer) is a core component of the 3GPP's ATSSS framework, standardized from Release 16 onward. It operates at the data link (Layer 2) and network (Layer 3) layers to manage user plane traffic for a single Protocol Data Unit (PDU) Session that is simultaneously connected to both a 3GPP access network (like 5G New Radio) and a non-3GPP access network (like trusted Wi-Fi). The primary architectural entity implementing ATSSS-LL functionality is the User Plane Function (UPF), which acts as a multi-access PDU Session Anchor. The UPF is equipped with an ATSSS-LL capability that includes classifiers and a steering function. The classifier, configured via control plane policies from the Session Management Function (SMF), inspects incoming uplink or downlink packets and assigns them to a specific access leg (3GPP, non-3GPP, or both) based on filters such as 5-tuple (source/destination IP/port, protocol). The steering function then executes the decided action: steering (sending all packets of a flow over one selected access), switching (moving an ongoing flow from one access to another), or splitting (distributing packets of a single flow across multiple accesses). For splitting, ATSSS-LL supports mechanisms like Multi-Path Transmission Control Protocol (MPTCP) at the transport layer or a generic 3GPP-defined Adaptation Layer (ATSSS-LL-A) that sits between the IP layer and the underlying access-specific link layers, adding sequence numbers for packet reordering. The UPF and the User Equipment (UE) must both support the same ATSSS-LL mechanisms to coordinate these operations. Performance measurements (e.g., latency, loss) from both accesses can be fed back to the control plane to dynamically adjust steering policies, optimizing for the best application performance. This low-layer integration allows for rapid, network-assisted decisions that are transparent to most higher-layer applications, providing a robust and efficient multi-access data plane.

Purpose & Motivation

ATSSS-LL was created to address the growing need for seamless and efficient utilization of multiple concurrent access technologies, a cornerstone of 5G system design. Prior to ATSSS, devices could connect to multiple networks (e.g., cellular and Wi-Fi) but typically used them in a mutually exclusive or static manner (like IP flow mobility), often relying on higher-layer, end-to-end solutions like MPTCP which required application support and were not network-controlled. This led to suboptimal resource use, inability to react quickly to network changes, and a fragmented user experience. The purpose of ATSSS-LL is to give the 3GPP network core (specifically the 5GC) direct, low-layer control over how a user's traffic is distributed across available access paths. This solves key problems: it improves aggregate throughput and capacity by bonding access links, enhances connection reliability and resilience through instant failover (switching) between accesses, and reduces latency by steering sensitive traffic to the best-performing path. It was motivated by the vision of Always-Best-Connected services and the need to support demanding use cases like ultra-reliable low-latency communications (URLLC) and enhanced mobile broadband (eMBB) where a single access might be insufficient. By operating at low layers and being managed by network policies, ATSSS-LL provides a standardized, optimized, and transparent multi-access service that is superior to pre-standard or purely client-based implementations.